1. Field of the Invention
The present invention relates to a global positioning system (GPS) and more particularly, to a method of improving the accuracy of differential phase measurements related to certain GPS receivers.
2. Description of the Prior Art
GPS receivers enable individuals to determine, with a given accuracy, their position on Earth or in space. These position measurements are generally based on signals received from a plurality of satellites. Historically, a GPS receiving system comprises an antenna which receives several signals, each from a different GPS satellite. Since the relationship between the signals received from the satellites is known, the GPS receiving system can compare the signals and note phase differences. These phase differences are then used to pinpoint the location of the GPS receiving system. A GPS receiving system, for example, can be a hand-held unit used by a military person or a mountain climber.
With the development of advanced space vehicles and other types of aircraft and navigable ships it has become more desirable to explore the use of GPS receivers as an indication of platform orientation; an attitude indicator, for example. Such a GPS receiver includes a plurality of antennas with a known orientation. For example, on the bow, stern, starboard and port locations of a ship. Such a system can be used in further research related to the performance of the ship and also as part of a navigational control system onboard the ship. For this type of GPS receiver one of the antennas is deemed the `master` antenna and the remaining antennas are deemed `slave` antennas. In operation, the GPS receiver tracks the carrier of the master antenna and notes the phase difference between the signal received by the master antenna with the signals received by the slave antennas. Thus, an orientation of a platform such as a space vehicle, an airplane, or a ship can be accurately calculated based on GPS. See U.S. Pat. No. 5,101,356 (Bowen et al.), for example. Such a system, however, is often expensive since it requires a hardware signal path for each antenna.
In response to alleviating this cost, GPS receivers have been developed to multiplex the incoming antenna signals, requiring only one main hardware path. A multiplexing GPS receiver is described in U.S. Pat. No. 5,268,695 (Dentinger et al.). In a GPS multiplexing receiver system the carrier wave signal received by the master antenna is tracked with a local oscillator such that the phase of the master antenna carrier is held near zero. One or more slave antenna signals are then received and the phase of these signals is compared with the local oscillator. Assuming that the master antenna tracking is perfect, the difference in phase between the local oscillator and the slave signals then defines the orientation of the receiving antennas. The process is continued and the antenna orientation information is updated.
One problem with such a system relates to the accuracy of acquiring and locking the local oscillator to the master carrier signal. After the local oscillator is locked to the master carrier signal the multiplexing GPS receiver then acquires signals related to the slave antennas. Once all of the slave antenna information is acquired the multiplexing GPS receiver then repeats the acquisition process by once again acquiring the master carrier signal. However, during the acquisition of the slave antenna information at least two sources of error can affect the ability of the multiplexing GPS receiver aster antenna tracking.
A first error is related to the local oscillator. Assuming that the platform which the master and slave antennas are attached is static, during acquisition of the slave antenna information the local oscillator may drift resulting in a differential phase measurement error since the drift will be considered a phase shift between a given slave antenna signal and the master antenna signal.
A second error is related to the dynamic motion of the platform. Typically the platform is not static, but rather is moving in three dimensional space. If the rotational speed about an axis of the platform, for example, is large enough such that the position of the master antenna substantially changes during the cycle of multiplexing through the master and slave antenna signals, the change in position of the master antenna during this acquisition cycle will result in a substantial differential phase measurement error. This dynamic error is directly dependent on the changing orientation of the platform.